Recently, many engines have plural and in particular two inlet ports for one cylinder.
It is known that, in order to ensure stability in combustibility particularly at the time of low loads, it is effective to cause air admitted to swirl in the cylinder, and many attempts have been made to improve inlet passages for formation of swirls. For instance, in an engine having two inlet ports for one cylinder, an independent inlet passage communicating with one of the inlet ports is provided with an opening/closing valve, while the other inlet port is disposed in a direction substantially tangential with the cylinder to allow air admitted to swirl therein. For such an engine, the opening/closing valve is closed at the time of low load to cause air to be admitted through the other inlet port only whereby strong swirls are formed. This leads to the fact that swirls caused to occur by air admitted from one inlet port are not prevented by blocking admission of air from the other inlet port which disturbs occurrence of swirls. At the time of high loads, of course, the valve is opened to supply a sufficient amount of air through the two inlet ports whereby adequate outputs are ensured.
For such engines as causing swirls using the opening/closing valve, an actuator for driving the opening or closing the valve and other devices should further be mounted leading to complexity in structural arrangements and raising costs of manufacturing.